Attempts are being made to apply a Co—Sn compound as a negative electrode material to lithium ion secondary batteries because Co—Sn compounds are expected to provide a higher capacity than graphite materials. Specific examples of a Co—Sn compound which are being applied are CoSn, CoSn2, and the like. In a lithium ion secondary battery using a Co—Sn compound as a negative electrode material, it is concerned that the cycle properties of the lithium ion secondary battery are degraded due to repeated expansion and contraction of the Co—Sn compound caused by charge and discharge.
Patent Document 1 discloses Co3Sn2 as an example of an alloy which exhibits a higher capacity than graphite materials and which at the same time has good cycle properties without causing excessive expansion and contraction. Patent Document 2 discloses Co3Sn2 as an example of an intermetallic compound which can be used in a negative electrode material. Patent Document 3 describes CoSn2 as an intermetallic compound which is used in negative electrode materials and discloses that Co3Sn2, which is formed as a by-product, is eliminated by heat treatment. Patent Document 4 describes CoSn2 as a Co—Sn compound for use in a negative electrode material, and it discloses that Co3Sn2, which is formed as a by-product other than the desired compound, is eliminated by heat treatment. Patent Document 5 discloses a Co—Sn—Fe compound as a negative electrode material and mentions using inexpensive Fe, which is an element cheaper than Co but having the same battery capacity as Co, as a means of decreasing the amount of expensive Co.